The present invention relates to a process for purifying telomers of the formula CF.sub.2 ClCFCl(CF.sub.2 CFCl).sub.n Cl, where n is in the range of 1 to 10. The telomers of this invention are saturated, low molecular weight polymers which are useful for preparing non-flammable hydraulic fluids.
Various methods of preparing chlorotrifluoroethylene ("CTFE") telomers are known in the prior art and have been practiced commercially for many years. An article by William T. Miller, Jr. et al in Industrial and Engineering Chemistry, pages 333-337 (1947), entitled "Low Polymers of Chlorotrifluoroethylene", describes a process for producing low molecular weight polymers of CTFE by polymerization in a solution of chloroform using benzoyl peroxide as a polymerization promoter. Other solvents disclosed in the reference as being useful for this purpose include carbon tetrachloride and tetrachloroethylene. The solution is heated in a pressure vessel for 13/4 hours at 100.degree. C., and the unreacted CTFE monomer and chloroform are removed by distillation, leaving a crude telomer of general formula CHCl.sub.2 (CF.sub.2 CClF).sub.n Cl, which can be further heated and distilled to yield products ranging from a light oil to a semi-solid wax or grease.
Another process for preparing low molecular weight CTFE polymers is described in U.S. Pat. No. 2,788,375, issued Apr. 9, 1957. This process comprises reacting CTFE with a saturated brominated compound in the presence of a source of radiation. Suitable brominated compounds include 1,2-dibromo-2-chlorotrifluoroethane (CF.sub.2 BrCClFBr). The saturated bromopolychlorofluoro compounds obtained by this process can then be distilled, and the isolated fractions reacted with chlorine to prepare polychlorofluoro compounds. The compounds are predominantly higher molecular weight telomers, i.e. n is greater than 4.
A more recent development in this field is described in a series of articles by Y. Pietrasanta et al entitled "Telomerization by Redox Catalysis" appearing in the European Polymer Journal, Vol. 12 (1976). This technology involves the reaction of single carbon halogenated telogens, such as CCl.sub.4 and CCl.sub.3 Br, with CTFE in the presence of benzoin and a suitable redox catalyst, such as ferric chloride. The telomerization reaction is suitably carried out in acetonitrile which is a common solvent for the reactants and catalysts. The telomerization reaction can be illustrated as follows: ##STR1## where X is chlorine or bromine. The reference further discloses that the use of CCl.sub.3 Br as a telogen results in a lower degree of telomerization and a higher proportion of monoaddition product than would occur with the use of CCl.sub.4.
The redox process has the advantage of directly preparing low molecular weight products without the necessity of cracking or fractionating a higher molecular weight polymer.
A modification of the redox process is disclosed in commonly assigned U.S. application Ser. No. 116,843, filed Nov. 5, 1987. This modification involves the telomerization reaction of chlorotrifluoroethylene with 1,1,2-trichlorotrifluoroethane in a nitrile group-containing solvent, such as acetonitrile, propionitrile and ethyl cyanoacetate, in the presence of a catalytic amount of metallic iron or stainless steel type 410-L, which is an alloy of 87.5% iron and 12.5% chromium. Optionally a halide-containing compound selected from the group consisting of LiCl, FeCl.sub.3, MoCl.sub.5, tetramethyl ammonium chloride, tetrabutyl ammonium bromide, triethylamine hydrochloride and n-chlorosuccinide, can also be added to the reaction mixture. The halide-containing compound functions as a chain-terminating agent during the reaction to limit the formation of higher molecular weight species. This process can be illustrated as follows: EQU CF.sub.2 ClCCl.sub.2 F+nCF.sub.2 =CFCl.fwdarw.CF.sub.2 ClCFCl(CF.sub.2 CFCl).sub.n Cl, (2)
where n is in the range of 1 to 10.
Reaction (2) results in the preparation of a mixture or distribution of individual telomer species having molecular weights corresponding to n values of from 1 to 10, rather than pure isomers having a discrete structure, i.e. a single n value. Separation of the individual telomer species from the mixture is accomplished by distillation using procedures well known to those skilled in this art.
Although telomers produced according to this latter process represent a significant advance over the prior art, such telomers have been found to contain minor amounts of impurities which can present corrosion problems when used in demanding applications such as nonflammable hydraulic fluids. An efficient and economically feasible purification process for such telomers would therefore be highly desirable.